In recent years, businesses using networks are rapidly expanded. For example, a system of purchasing items at on-line stores on the network is widely used. In most of such on-line stores, a system of recommending items to users is provided. For example, when the user accesses to detailed information of a certain item, information of items relating to the item are presented as related items or recommended items to the user. Such system is realized by, for example, using a cooperative filtering method described in JP-A-2003-167901 (Patent Document 1). The cooperative filtering method is a recommendation method using information (for example, purchase history and so on) of users having similar taste.